Conventional front suspension systems, such as found in light and medium duty pickup trucks, are typically comprised of steel coil springs, shock absorbers, steel roll stabilizer bar, and a 5-bar linkage, or similar arrangement. The rear suspension is typically comprised of steel multi-leaf springs and shock absorbers, or similar arrangement.
While such suspension configurations have very acceptable on-road performance for conventional light and medium pickup trucks, their off-road mobility is hampered by the relative lack of axle roll or tramp articulation when traversing uneven terrain. The front stabilizer bar stiffness and the rear leaf spring stiffness prevent the axles from achieving full tramp movement from the jounce travel limit to the rebound travel limit alternatively side to side, resulting in loss of ground contact when one wheel is traversing a large obstacle, such as a rock or boulder.
For maximum mobility and control over sand, snow, low friction surfaces, and rugged off-road terrain, it is desirable to maintain ground contact with all four tires at all times. Increasing the front and rear axle tramp compliance increases the ability of the tires to maintain ground contact under off-road conditions. However, increasing axle compliance by reducing the axle tramp stiffness to achieve more favorable off-road axle articulation and mobility is limited by deteriorated ride and handling characteristics when the vehicle is driven at higher speeds on normal road surfaces. Off-road mobility is further inhibited by the reduction in ground clearance which occurs when the vehicle is loaded.
Thus, a dual mode suspension system that provides the ability to alter suspension characteristics to optimize off-road mobility while maintaining the desired suspension characteristics for on-road driving conditions is advantageous. A dual mode suspension system consists of two operating modes, in the present embodiment, an on-road driving mode and an off-road driving mode. The on-road driving mode is configured to provide the desired roll stiffness, ride stiffness, and front to rear roll couple distribution for on-road driving. For the off-road driving mode, the system is reconfigured to provide the desired lower front and rear roll (or tramp) stiffness and increased tramp compliance, resulting in greater axle articulation for off-road driving. A dual mode suspension may also be utilized for other vehicle applications where two different suspension characteristics are desired, such as one mode for normal driving with ride quality optimized, and the other mode for more responsive handling, with firmer suspension settings.
However, there is a need for a mechanism for switching to the appropriate suspension mode in a dual mode suspension system. One possibility is to provide a manually activated switch accessible by a driver of the vehicle. However, this mechanism requires action and knowledge by the driver as to which mode to engage and when to do so, which could result in inappropriate vehicle ride and handling characteristics.
Another method for switching to the off-road driving mode may involve automatically doing so when the vehicle is placed into a four-wheel drive mode. However, this method requires the driver to affirmatively shift into four-wheel drive mode when driving off-road to activate the off-road driving mode and the off-road driving mode would never be activated when driving in an off-road condition while in two-wheel drive.
Thus, there exists a need for automatically shifting between an on-road driving mode and an off-road driving mode under appropriate circumstances without any undesirable consequences.